Moreover, the invention relates to a deammonifying installation for treating wastewater containing ammonia, having at least one aeration tank and at least one hydrocyclone for separating sludge from the aeration tank into a specifically heavy fraction containing mostly anaerobic ammonium-oxidizing bacteria (anammox) and into a specifically light fraction, whereby the hydrocyclone has an inlet that is flow-connected to the aeration tank and that serves to feed in the sludge, an underflow that is flow-connected to the aeration tank and that serves to return the separated specifically heavy fraction to the aeration tank, and an overflow that serves to discharge the separated specifically light fraction from the hydrocyclone.
The activated sludge process is a process for biological wastewater treatment in wastewater treatment plants. Here, the usually municipal wastewater is virtually rid, that is to say, cleaned of organic impurities through the metabolic activity of aerobic chemoorganoheterotrophic microorganisms, the so-called activated sludge. The process begins after the separation or settling of the coarse fractions that are dewatered, separated, digested and incinerated. For municipal wastewater, this process is one of the classic intensive treatment processes. An advantage is the general applicability and good cleaning effect for wastewater in order to reduce the suspended matter content, the chemical oxygen demand (COD), the five-day biochemical oxygen demand (BOD5), and the nitrogen compounds (N).
Installations that run on the basis of the activated sludge process can be operated continuously, that is to say, in uninterrupted operation (conventional activated sludge installation), as well as discontinuously (Sequencing Batch Reactor—SBR installation). Moreover, there are also so-called membrane activated sludge installations in which a membrane is used to separate the treated water from the sludge. All of these variants have in common the fact that bacteria mass or biomass that is suspended in water and that is also referred to as activated sludge takes over the biological cleaning of the wastewater. For this purpose, each installation has at least one aeration tank in which the wastewater is mixed with the activated sludge and is thus brought into intensive contact with the activated sludge.
The biomass formed in the aeration tank during the aerobic biological wastewater treatment by means of the degradation of the constituents present in the wastewater is referred to as activated sludge. It consists mainly of bacteria, fungi, protozoa, EPS and other constituents. Microscopic examinations confirm that activated sludge flocs of bacteria and protozoa are “activated”. This is why they are called activated sludge. During the technical utilization in the activated sludge process, the activated sludge is usually present in the form of activated sludge flocs that, in addition to living and dead biomass, also contain adsorbed and embedded organic compounds and minerals.
In the activated sludge process, after the contaminants in the wastewater have been degraded by activated sludge, this sludge is separated from the cleaned water in the so-called secondary treatment stage. Most of the separated sludge is returned to the aeration tank as returned activated sludge or recirculated activated sludge. This ensures that the activated sludge concentration in the aeration tank can be maintained. The activated flocs contained in the return activated sludge renew the cleaning capacity of the activation process. The non-recirculated, smaller volume flow of activated sludge is called the surplus sludge. In other words, the surplus sludge is the portion of the activated sludge that is withdrawn and pumped into the sludge treatment stage in order to keep the desired biomass concentration constant. As a rule, this removed biomass growth is fed, together with the primary sludge, to the sludge digestion stage and finally to the sludge dewatering stage.
Nowadays, conventional wastewater treatment plants, almost exclusively make use of biological nitrification/denitrification for purposes of nitrogen elimination. The term nitrogen elimination refers to the conversion of biologically available nitrogen compounds such as ammonia (NH4), nitrite (NO2) and nitrate (NO3) into elementary nitrogen (N2), which gases out into the ambient air as a harmless end product. During the nitrification, ammonia is oxidized by oxygen via the intermediate product nitrite to form nitrate. During the subsequent denitrification, the nitrate is reduced in a first reduction step to form nitrite and in a second reduction step to form nitrogen.
Biological nitrification/denitrification has the drawback of a high oxygen demand and thus a high energy consumption. Moreover, during the denitrification, organic carbon is consumed, which has a negative effect on the further cleaning process and on the sludge properties.
With deammonification, as compared to nitrification/denitrification, only 40% of the oxygen is needed or the energy consumption for the nitrogen elimination is reduced by 60%. Deammonification is an autotrophic process in which no organic carbon is needed. Consequently, the rest of the cleaning process becomes more stable.
Deammonification is an efficient process for biological nitrogen elimination, for example, also in the case of wastewater streams with high ammonia concentrations. Two bacteria groups are involved in biological deammonification with suspended biomass: on the one hand, the aerobic ammonia-oxidizing bacteria (AOB), which convert ammonia into nitrite and, on the other hand, the anaerobic ammonia-oxidizing and elementary-nitrogen-producing bacteria (anammox), especially Planctomycetes, which carry out this step using the previously produced nitrite.
Relative to the mass conversion, the aerobic ammonia-oxidizing bacteria (AOB) produce ten times more new bacteria mass than the anaerobic ammonium-oxidizing bacteria (anammox). Therefore, the retention time of the sludge in a single-sludge system has to be at least so long that the slow-growing anaerobic ammonium-oxidizing bacteria (anammox) can become established.
Methods for one-stage or two-stage deammonification are already quite well known, for example, from international patent application WO 2007/033393 A1 or European patent specification EP 0 327 184 B1.
A drawback here is particularly the much longer generation times for the anaerobic ammonium-oxidizing bacteria (anammox), which is longer by a factor of 10 than those for the aerobic ammonia-oxidizing bacteria (AOB). As a result, a stable system can only form when the retention time of the sludge or the bacteria in the tank is sufficiently long. This, in turn, calls for large reaction volumes and correspondingly dimensioned tanks.
Moreover, an adequately high wastewater temperature (>25° C. [77° F.]) is the basis for the existence or growth of the anaerobic ammonium-oxidizing bacteria (anammox). Heating up the wastewater, however, requires a great deal of resources in terms of energy, which is why the processes described are not cost-effectively feasible or useful for wastewater that is at a low temperature.
Moreover, the presence of groups of bacteria (NOB) that convert the formed nitrite into nitrate under aerobic conditions has proven to be disadvantageous. This group of bacteria has generation times that are shorter by a factor of 10 than those of the anaerobic ammonium-oxidizing bacteria (anammox). In order to compensate for these different generation times, it has already been considered to carry out the aerated phase of the single-sludge system at a very low oxygen level (<0.4 mg O2/l). With this approach, the nitrate-forming bacteria (NOB) have little or no oxygen available for converting the nitrite which, in turn, is very advantageous for the anaerobic ammonium-oxidizing bacteria (anammox). The reduced oxygen supply during the aerated phase, however, has the drawback that the aerobic conversion of the ammonia into nitrite also transpires under oxygen-limited conditions and consequently, takes place very slowly.
The slow-growing Planctomycetes, which have a generation time that is longer by a factor of 10 than the nitrite-forming bacteria (AOB), have the special property that very many individual bacteria form a spherical conglomerate, so-called Planctomycete granules. These Planctomycete granules have a very high density (1010 bacteria/ml).
In addition to containing the ammonia that is to be degraded, the wastewater to be treated also contains organic substances such as organic acids and other organic substances that are described by the sum parameter “dissolved COD” and that can have values of several hundred mg/l (typically 100 to 2000 mg/l). These organic substances are degraded by very fast-growing heterotrophic bacteria. The heterotrophic bacteria often settle on the Planctomycete granules and coat them with an organic cover layer or covering. The cover layer limits diffusion, thereby making the conversion of ammonia (NH4) and nitrite (NO2) into elementary nitrogen (N2) more difficult, since the substrate (NH4 and NO2) first has to pass through this cover layer before it is available to the Planctomycetes for the conversion.
The wastewater that is to be treated, often wastewater from sludge digestion (anaerobic stabilization of sludge) or general wastewater with a high nitrogen concentration, also often contains—in addition to ammonia (NH4) and organic degradable substances—inorganic substances such as, for instance, calcium carbonate and/or struvite, which can likewise be deposited onto the surface of the Planctomycete granules. Suspended matter present in the wastewater, which can amount to several hundred mg/l (typically 50 to 1000 mg/l), form or enlarge the cover layer on the Planctomycete granules.
Due to the diffusion-limiting effect, the cover layer on the Planctomycete granules leads to a massive drop in the capacity of the deammonifying installation. Comparative measurements between exposed, uncovered Planctomycete granules and granules with a cover layer have shown a difference in the specific conversion rate of nitrogen (mg H/g TS) by a factor of 4 to 6.
The deposits or cover layers on the Planctomycete granules can already be seen with the naked eye. Exposed, uncovered granules are an intensive red/rust-red color, and the granules coated by a cover layer are slightly reddish/brown to dark brown in color, depending on the extent to which they are coated.
European patent specification EP 2 163 524 B1 has already described a method and a deammonifying installation of the above-mentioned type. In the disclosed method, in order to keep the biomass concentration in the installation constant, the surplus sludge that is withdrawn from the installation is not disposed of and transported to the sludge digestion stage, but rather, it is fed into a hydrocyclone where it is separated into a specifically heavy fraction (underflow) and a specifically light fraction (overflow). In this process, the density differences between the two bacteria groups (anammox and AOB) present in the surplus sludge are used to separate the surplus sludge into a heavy phase containing mostly the anaerobic ammonium-oxidizing bacteria (anammox), and a light phase (AOB). By returning the bacteria group (anammox) contained in the heavy phase to the aeration tank of the installation, the slow-growing bacteria group (anammox) becomes established in the aeration tank.
The two sludge fractions that are to be separated, namely, the specifically light fraction and the specifically heavy fraction, differ from each other markedly in terms of their density as well as in terms of their biological characteristics. These are completely different groups of bacteria. The Planctomycete granules consisting of several individual bacteria have a much greater density than the aerobic ammonia-oxidizing bacteria (AOB) that are present in flocculent form. Due to the density differences that exist between the two bacteria groups, the discharged surplus sludge can be separated into a heavy phase containing the Planctomycete granules and a light phase containing the flocculent sludge fraction. Owing to the density differences, the Planctomycete granules are considerably heavier than the flocs.